1. Field of the Invention
The present invention relates to image sensor pixels, and more specifically to pixels comprising elements formed in a semiconductor substrate such as a silicon wafer.
2. Discussion of the Related Art
An example of a pixel formed at the surface of a semiconductor substrate is described in U.S. Pat. No. 6,448,596. The pixel comprises a photodiode and an N-channel MOS transistor (NMOS). As illustrated in FIG. 4A of this patent, reproduced in FIG. 1 of the present application, the transistor comprises a ring-shaped gate 59, an N-type source area 56 formed at the substrate surface, inside of the gate, and an N-type drain area 57a formed around the gate. The photodiode is formed of an extension 57 of the transistor drain area, of a well area 54a placed under extension 57, and of a buried layer 52a placed under this well area.
Incident photons arriving through an opening 63 of a reflective metal layer 62 placed above the pixel generate electron-hole pairs at the level of well area 54a. The holes thus formed are collected under the gate of the NMOS transistor in heavily-doped P-type pockets 55. In a read phase, the modifications of the electric conduction characteristics of the transistor are determined according to the quantity of holes stored under the gate. The holes accumulated under the gate are regularly “evacuated” by application of high voltages on the gate and on the transistor source/drain areas.
Such a pixel takes up a significant substrate surface area, especially due to the presence of the photodiode. Further, the pixels are insulated from one another by N-doped semiconductor areas 53 surrounding the pixels, which areas also increase the sensor surface area.